The present invention relates to magnets for medical magnetic resonance studies and more particularly to such magnets which comprise a ferromagnetic yoke as part of the magnet structure.
Medical magnetic resonance (MR) studies are typically carried out in strong magnetic fields greater than one kilogauss. In addition to a strong magnetic field, medical magnetic resonance studies typically require a magnetic field homogeneity of the order of a few parts per million. Considerable effort has been invested in improving magnets for medical MR with a goal toward achieving the field strengths required while attaining the necessary field homogeneity over a sufficiently large spatial volume in a structure that is clinically acceptable and commercially feasible.
One technique for improving magnet efficiency is to incorporate within the magnet structure a ferromagnetic yoke which not only comprises part of the structural support but which also defines a magnetic flux return path. The use of ferromagnetic return paths in medical magnetic resonance scanner magnets is disclosed in U.S. Pat. No. 4,675,609 to Danby et. al. Ferromagnetic yoke structure is likewise disclosed in U.S. Pat. No. 4,672,346 to Miyamoto et. al. In addition to improving efficiency, the incorporation of a ferromagnetic flux return path can also be used to eliminate strong leakage magnetic fields which are inherent in aircore solenoidal magnets.
It would be desirable to incorporate ferromagnetic yokes in medical MR magnets having a strong magnetic field. The stronger the magnetic field developed by the magnet, however, the more difficult it is to achieve a magnet structure which would be considered practical by the medical community for clinical use.
To avoid magnetic saturation of the ferromagnetic yoke at high field strengths, the dimensions of the yoke cross sections along the flux return path become substantial. Greater yoke cross-sectional area results, of course, in an increase in magnet weight.
Additionally, larger yoke structures can result in obstructions which hinder easy access to and egress from the patient gap of the magnet where a patient is situated during magnetic resonance scanning. Any compromises to the required yoke design from the standpoint of flux return path reluctance, in order to accommodate patient access, can materially increase the magnetic leakage field and reduce the homogeneity of the magnetic field within the gap. Another desirable feature in a medical MR magnet is a large gap for receiving the patient to be studied. A large gap facilitates patient positioning and permits large patients to be studied by magnetic resonance. A related but distinct consideration which is impacted by gap size is that of access to a patient by medical personnel while the patient is within the gap.
Yet another desired improvement to medical MR magnets is the provision of features or means to suppress the generation of eddy currents, particularly within the poles of the magnet. Most medical MR scanning techniques in use at this time involve the use of time-varying magnetic fields, usually in the form of pulsed magnetic field gradients. These time-varying magnetic fields may induce eddy currents in conductive parts of the magnet, and such eddy currents will in turn generate magnetic fields which can degrade magnetic field stability during data acquisition and field homogeneity of the magnet. suppression of eddy currents is therefore highly desirable.
Accordingly, it is an object of the invention to provide a magnet having a ferromagnetic yoke with a strong field for use in medical magnetic resonance studies which has a large gap and a structure providing open entry to the gap. Moreover magnets currently employed today to generate high field MRI most generally employ air-core superconductors and do not utilize ferromagnetic structures to concentrate field in the imaging region. Such air-core magnets that fail to exploit the benefits of ferromagnetic flux concentration are fundamentally inefficient as compared to ferromagnetic core magnets and require as much as eight times as many ampere-turns to achieve the same center field as a ferromagnetic core (e.g. iron core) electromagnet (e.g. iron core superconducting magnet) as covered by U.S. Pat. No. 4,766,378, for Nuclear Magnetic Resonance Scanners and commonly assigned herewith.
It is another object of the invention to provide a magnet having a ferromagnetic yoke for medical magnetic resonance scanning and having a large patient gap, and selected dimensions, geometry and materials in order to improve the magnet.
It is another object of the invention to provide a magnet for medical magnetic resonance studies which has a ferromagnetic yoke with a structure that provides open entry to the patient gap of the magnet, together with structure for eddy current suppression.
It is another object of the invention to provide a magnet and MR apparatus that would be suitable to function in a surgical operating room environment to provide images for MRI guided surgery.